This invention relates to a mobile communication system and, in particular, to a soft-handover technique between base station controllers of the mobile communication system.
Conventionally, a handover technique is used in a mobile communication system to prevent the communication from being interrupted when a mobile terminal is moved among cells. A soft-handover technique is known as one type of the handover technique and indispensable to a CDMA communication system.
In general, the soft-handover technique is applied to each mobile service switching center (or mobile communication exchange) of the mobile communication system. In other words, each mobile service switching center independently performs the soft-handover. Thus, a hard-handover technique is used when the mobile terminal moves from an service area covered by the mobile service switching center to another service area covered by another mobile service switching center. The hard-handover brings about problems, such as instantaneous disconnection of the communication, deterioration of speech quality of the communication, etc.
A conventional soft-handover technique for performing of the soft-handover between base stations connected to different mobile service switching centers is disclosed in Japanese Unexamined Patent Publication No. Hei 09-505948. The soft-handover technique is used in a mobile communication system which comprises mobile communication exchanges, base stations, and at least one mobile terminal.
In the mobile communication system, each of mobile communication exchanges can perform the soft-handover both between base stations connected to it and between base stations connected to it and to another mobile communication exchange. Therefore, throughout a whole cellular service area, a seamless soft-handover with a signal diversity synthesis can be achieved between the base stations connected to different mobile communication exchanges.
For example, it is assumed that the mobile terminal is located in a first cell covered by a first base station connected to a first mobile communication exchange and communicates with the other system user (or terminal) through the first base station and the first mobile communication exchange. The first mobile communication exchange receives user information signals sent from the mobile terminal through the first base station and performs the signal diversity synthesis of the received user information signals to form a synthesized user information signal and to send the other system user the synthesized user information. Under this situation, when the mobile terminal moves from the first cell to a second cell adjacent to the first cell and covered by a second base station connected to a second mobile communication exchange, connection between the mobile station and the second mobile communication exchange is established through the second base station. The second mobile communication exchange relays user information signals, which is sent from the mobile station and which is received by the second base station, to the first communication exchange. The first mobile communication exchange carries out signal diversity synthesis of the user information signals received by the first and the second base station to transmit a new synthesized user information signal to the other system user.
However, the conventional soft-handover technique can not applied to a larger mobile communication system, such as a mobile communication system having base station controllers between mobile communication exchanges and the base stations.
Moreover, the conventional soft-handover technique requires a line or channel via the mobile communication exchanges because the handover is performed between the mobile communication exchanges. It is therefore necessary to provide another device for synthesizing speech signal and data signal. This leads to the problem that the system is increased in scale.
Furthermore, each of the mobile communication exchanges must store cell ID analysis information for the whole system.
Another conventional soft-handover technique for performing of the soft-handover between base stations connected to different mobile communication exchanges is disclosed in Japanese Unexamined Patent Publication No. Hei 10-145834. The soft-handover technique is applied to a mobile communication system which comprises mobile communication exchanges, base stations, at least one mobile terminal, and lines each of which connects between two base stations adjacent to each other. The lines are logically independent of the mobile communication exchanges.
In the mobile communication system, traffic information signals are exchanged between two base stations adjacent to each other to carry out the soft-handover when the mobile station moves between two cells covered by the two base stations. The soft-handover can be carried out even if the base stations are connected to different mobile communication exchanges.
However, the leading base station, which is one of the two base stations and leads the soft-handover, is changed from one to the other with the movement of the mobile station, according to the soft-handover technique. This certainly brings necessity of a hard-handover when the soft-handover is carried out between the base stations connected to the different mobile communication exchanges. Consequently, the soft-handover can hardly decrease frequency of the hard-handover.
It is therefore an object of this invention to provide a mobile communication system capable of achieving soft-handover between base station controllers connected to different mobile communication exchanges.
It is another object of this invention to provide a mobile communication system capable of achieving soft-handover without increasing the scale of the system and without storing cell ID analysis information of the whole system in each of base station controllers.
It is still another object of this invention to provide a mobile communication system capable of decreasing frequency of hard-handover.
Other object of this invention will become clear as the description proceeds.
On describing the gist of this invention, it is possible to understand that a mobile communication system includes a plurality of base stations having cells for being connected to a mobile terminal by radio and a plurality of base station controllers connected between the base stations and a mobile exchange.
According to the gist of this invention, the mobile communication system comprises control signal lines connected among the base station controllers for translating control signals to perform soft-handover and to put concerning base station controllers into a soft-handover state regardless of the mobile exchange when the mobile terminal moves between cells. Connection lines is connected among the base station controllers for keeping communication between the mobile terminal and another terminal in the soft-handover state. Each of the base station controllers keep transmitting/receiving communication signals to/from the mobile exchange regardless location of the mobile terminal, when the communication starts through it.
In the mobile communication system, the base stations broadcast predetermine radio waves over the cells, respectively. The mobile terminal detects the predetermined radio waves to produce a field strength report signal representative of the predetermined radio waves. Each of the base station controllers comprises a soft-handover control section to perform the soft-handover on the basis of the field strength report signal. A conversion section converts the communication signals between a first type for the base stations and a second type for the mobile exchange. A switching section is connected to the soft-handover control section, the conversion section, predetermined base stations of the base stations, the predetermined connections lines of the base stations, and predetermined control signal lines of the control signal lines for switching paths among them.
According to another gist of this invention, a method of performing a soft-handover in a mobile communication system comprises the steps of receiving at a first base station controller a field strength report signal transmitted by a mobile terminal which communicates with another terminal through the first base station controller. A step of establishing establishes a connection on a connection line connected between the first base station controller and a second base station controller which is connected to a base station indicated by the field strength report signal.
According to still another gist of this invention, a mobile communication system includes first and second base stations having first and second cells adjacent to each other, respectively. The first and the second base stations are connected to a mobile terminal by radio. First and second base station controllers is connected to the first and the second base stations, respectively. The first and the second base station controllers connect the first and the second base stations with a first mobile exchange. The mobile terminal transmits/receives communication signals to/from another terminal through the first base station, the first base station controller, and the first mobile exchange when the mobile terminal locates in the first cell. The mobile communication system comprises a control signal line connected between the first base station controller and the second base station controller to perform soft-handover between the first base station controller and the second base station controller without respect to the first mobile exchange and to put the first base and the second base station controllers into a soft-handover state when the mobile terminal moves from the first cell to the second cell. A connection line is connected between the first base station controller and the second base station controller to translate the communication signals between the first base station controller and the second base station controller in the soft-handover state and to keep the first base station signal controller transmitting/receiving the communication signals to/from the first mobile exchange.
In the mobile communication system, the first and the second base stations periodically broadcast predetermined radio waves. The mobile terminal detects electric field strength of the predetermined radio waves to produce a field strength report signal representative of the electric field strength. Each of the first base station controllers comprises a soft-handover control section to perform the soft-handover on the basis of the field strength report signal. A conversion section converts the communication signals between a first type for the first and the second base stations and a second type for the first mobile exchange. A switching section is connected to the soft-handover control section, the conversion section, the first or the second base station, the connections line, and the control signal line to switch paths among them.
According to further still another gist of this invention, a mobile communication system includes a plurality of mobile terminals, a plurality of base stations each of which has a cell and is connected to each of the mobile terminals through radio communication wave, a plurality of base station controllers each of which is connected to predetermined base stations of the base stations through communication lines to carry out transmission and reception of control signals, speech signals, and data signals to and from the predetermined base stations through the communication lines, and a mobile communication exchange which is connected to each of the base station controllers through a communication line and which controls exchanging operation with the base station controllers, the system performing soft-handover between the base station controllers when the mobile terminal is moved between two cells. The mobile communication system comprises a connection line as a communication line and a control signal line for transmission of control signals both of which connect the base station controllers.
In the mobile communication system, each of the base station controllers comprising a soft-handover control section which establishes the connection line with reference to a received electric field strength at said mobile terminal. A QCELP/PCM conversion section synthesizes QCELP signals of the speech signals and the data signals transmitted/received to and from the mobile terminal via the base station to produce a synthesized signal and converting the synthesized signal into a PCM signal. A switching section carries out path-connection between the communication line connected to the base station, the soft-handover control section, the QCELP/PCM conversion section, the connection line, and the control signal line. The soft-handover control section is connected to a soft-handover control section in another base station controller through the switching section and the control signal line.